Hydrogel Separator Membrane to Preserve Soil Microbial Fuel Cells Operation Under Hydric Stress

Abstract

The present study demonstrated one new concept in single-compartment soil microbial fuel cells (s-MFCs) to save tap water irrigation required to preserve the battery working for prolonged periods. The study employed poly(N-isopropylacrylamide) and poly(vinyl alcohol) as thermosensitive and super-hydrophilic hydrogels to retain the soil humidity. Under soil moisture stress, caused by natural or forced drying conditions (three scenarios tested), the battery operation autonomy gradually decayed, at a lower speed than MFC single-chamber without hydrogel membrane. In extreme heating conditions, i.e., reaching temperatures above the PNIPAAm/PVA LCST (32–33 °C), even though both cells had similar soil volumetric humidity of 14%–17% after 28 days of device operation, the hydrogel-based s-MFC reached a maximum power density of 0.071 μW/cm² and current density of 1.24 μA/cm², whereas the pristine cell remained with values relatively inferior (0.043 μW/cm² and current density of 0.88 μA/cm²). The sensors installed in the soil, close to the separator hydrogel membrane, proved that temperature, humidity and volumetric conductivities were stable over 28 days in the hydrogel-based biobattery. Altogether, the superabsorbent hydrogel copolymer showed a capability for a gradual battery hydration, which represents a step-forward in s-MFC field for future investigations with such thermoresponsive materials.